Interferometric measurement of positions, position changes, and physical quantities derived therefrom

ABSTRACT

Described is a method of interferometric measurement of positions and position changes, as well as physical quantities derived therefrom, of a part to be tested using heterodyne interferometry, with a laser being modulated to change the frequency of the radiation emitted by it using a time-variable pulsating injection current in order to generate the heterodyne frequency, and one portion of the emitted radiation is routed via an optical bypass, while the other portion is routed without the optical bypass to the part and, from there, to a measuring receiver. Improved evaluation of the measurement results is achieved with smaller dimensions due to the fact that the signal shape of the injection current has a rising edge that is steep compared to its pulse length and a subsequent plateau.

FIELD OF THE INVENTION

The present invention relates to a method of interferometric measurementof positions and position changes, as well as of physical quantitiesderived therefrom, of a part to be tested, using heterodyneinterferometry, with a laser being modulated to change the frequency ofthe radiation emitted by it using a time-variable pulsating injectioncurrent, in order to generate the heterodyne frequency, and portion ofthe emitted radiation is routed via an optical bypass, while the otherportion is routed without the optical bypass to the part and therefromto a measuring receiver.

BACKGROUND INFORMATION

Such a method is known in conjunction with different optical systems andis described, for example, in European Patent No. 420 897 B1. In orderto generate the heterodyne frequency, which is well-suited for thequantitative evaluation, the radiation emitted by the semiconductorlaser (laser diode) is modulated using a time-variable injectioncurrent. The radiation emitted by the laser is divided into two beams,one of which is routed via an optical bypass and the other is routedwithout an optical bypass to the part to be tested. The optical bypassis implemented, for example, using deflecting mirrors or a light guideloop. Details of heterodyne interferometric measurement can be found inthe aforementioned document.

Other embodiments for heterodyne interferometric measurement ofpositions, position changes, rotation angles, speeds and other physicalquantities derived therefrom are known, with the optical bypass beingimplementable using a bypass prism. With these methods and measuringdevices, positions or path differences, as well as quantities derivedtherefrom, can be measured with high accuracy, for example, in the nmrange. To modulate the laser, the signal of the injection current inthese known methods and devices has a sinusoidal, triangular or sawtoothshape with a rising edge that is flat compared with the pulse period,since only such signal shapes are considered suitable for obtainingreliable measurement results. In these methods, the typical length ofthe optical bypass, for example, for a heterodyne frequency in the MHZrange as customarily used, is on the order of a few decimeters, forexample, 40 cm. This relatively long optical bypass runs counter to thedesired miniaturization of measurement systems. In addition, byincreasing the length of the optical bypass, the contrast of theinterference pattern to be evaluated, which is required for theevaluation and should be as high as possible, is diminished, as can beseen from the coherence function, which shows the drop in contrast withincreasing length of the optical bypass AL in the form of an exponentialfunction. In this case, evaluation is made difficult by back reflexes ofthe optical system onto the laser diode, which results in the otherwisesingle-mode operation of the laser becoming multimode with a peak-shapedcoherence function being obtained and the exponential function being theenvelope and dropping more steeply than in single-mode operation. Inorder to avoid back reflexes, an isolator, for example, must be locatedupstream from the laser, which results in further expenses.

In another measurement method, i.e, a spectroscopic measurement ofexhaled air, it is known from Lachish et al., “Tunable diode laser basedspectroscopic system for ammonia detection in human respiration,” theReview of Scientific Instrument, Vol. 58, No. 6, June 1987, pp. 923-927,that a semiconductor laser can be controlled using a rectangularmodulation current to detect a null signal during consecutive lightpulses. The semiconductor laser is temperature stabilized in thismethod.

U.S. Pat. No. 4,765,738 proposes that, in order to measure the frequencyresponse of an optical receiver system, a heterodyne frequency begenerated using a laser diode and an optical bypass, with a rectangularmodulation being performed, among other things, in order to control asemiconductor laser. Frequencies up to 10 GHz are to be measured withthis method. An optical fiber length of 20 km is proposed.

SUMMARY OF THE INVENTION

The object of the present invention is to improve on the method so thatthe measurement results are improved with a simpler and miniaturizedmeasuring system.

This object is achieved with the signal shape of the injection currentwhich has a steep rising edge compared to its pulse length and asubsequent plateau. Surprisingly, this laser control having steep risingedges and a subsequent plateau, contrary to the usual laser control usedfor modifying frequencies, results in improved heterodyneinterferometric measurement with the optical path diminishable to lessthan 1 cm. Thus the coherence function of the interference signalcontrast can be substantially improved, resulting in a measurementsignal that is easier to evaluate. At the same time, the dimensions ofthe system are considerably reduced and, for example, a considerablysmaller bypass prism can be used to miniaturize the measuring system.This is an important advantage, for example, in the case ofmultidimensional measurements with a plurality of measuring channels,such as those performed with a multi-axis vibrometer. A test model hasshown that the dimensions can be reduced several times compared toconventional measurement systems. As an additional advantage, the systemcan be made insensitive to back reflexes, with the length of the opticalbypass being accurately set within narrow tolerances, so that exactevaluations are obtained even with a multimode interference signal inthe region of a peak of the coherence function without the need forelaborate measures to suppress back reflexes. Narrow tolerances of theoptical bypass can be easily observed with the small overall length ofthe optical bypass, which is on the order of 1 mm, for example. In orderto eliminate the instability of the interference signal at the steeppulse edges during evaluation, the signal converted in a photoelectrictransducer of the measuring receiver is advantageously evaluated with adelay during the plateau, only after the edge of the injection currentappears.

A simple signal shape is, for example, rectangular pulses of theinjection current, which results in extremely steep edges compared tothe total pulse length. Relatively steep edges of the signal shape can,however, also be obtained using trapezoidal pulse shapes, pulse shapesthat are sinusoidal at the edges, or similar shapes with a plateau.

One advantageous mode of operation is when the injection currentpulsates between a minimum value that is less than the threshold currentof the laser and a maximum value that is greater than the thresholdcurrent. Thus no radiation is emitted during the pauses between pulses.As an alternative, the injection current may pulsate between a minimumvalue that is greater than the threshold current of the laser and agreater maximum value. In this operating mode, the laser continuouslyemits radiation whose frequency varies.

In order to eliminate the instability of the interference signal at thesteep pulse edges during evaluation, the signal converted in aphotoelectric transducer of the measuring receiver is advantageouslyevaluated with a delay during the plateau, only after the edge of theinjection current appears.

The method can be advantageously used so that a plurality of such lasersignals are related to form a plurality of measurement channels intime-multiplex mode, with the heterodyne signals of the differentmeasurement channels being generated in non-overlapping time windows.The different channels can be easily evaluated separately usingmultiplex mode in the evaluation circuit. For simple design and simpleevaluation, the laser control signals are advantageously delivered by acommon control circuit, and laser diodes, each assigned to one measuringchannel, are provided, and a photoelectric transducer that is common tothe measurement channels is provided, from which the signals arereceived in the time multiplex mode and are evaluated separately foreach measurement channel in a downstream evaluating circuit.

In one important application the measurement channels are assigned todifferent dimensions. Thus positions and position changes over time canbe easily measured and evaluated as in a multi-axis vibrometer, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a variation over time of an injection current in a firstoperation mode.

FIG. 2 shows another variation over time of the injection current in afirst operating mode.

FIG. 3 shows a time-multiplex heterodyne method.

FIG. 4 shows a signal shape of the injection current of a measurementchannel in the time-multiplex heterodyne method according to Figure 3.

DETAILED DESCRIPTION

FIG. 1 shows the variation over time t of the injection current I_(i).Injection current I_(i) has a rectangular signal shape, i.e., aparticularly steep rising edge and a subsequent plateau for each pulse.The maximum of injection current I_(i) is I_(i) that is greater thanthreshold current I_(th) of a laser diode, while the minimum injectioncurrent I₀ is less than threshold current I_(th). With this injectioncurrent I_(i) modulated in a rectangular shape, a frequency shift isobtained that is surprisingly well-suited for generating the heterodynefrequency, with a shorter optical bypass, on the order of millimeters,being sufficient for forming the phase shift between a first and asecond partial beam emitted by the laser, which is required forgenerating the heterodyne frequency.

For the design of the overall measurement system for the heterodynemethod using the frequency modulated laser and the optical bypass,reference is made to the aforementioned European Patent No. 420 897 B1and other related art for such measuring systems; optical bypasses inthe form of bypass prisms are also known. In these known measurementsystems used in the heterodyne method, laser modulation according to thepresent invention can be used and thus the length of the optical bypasscan be substantially shortened, so that not only a substantially morecompact design, but also improved measurement evaluation, can beachieved.

FIG. 2 also shows an injection current I_(i) modulated in a rectangularshape; contrary to FIG. 1, the minimum injection current I₀, however, isgreater than threshold current I_(th), so that radiation is emitted bythe laser even during the pulse pauses, which, however, isfrequency-shifted with respect to the radiation emitted during therectangular pulse.

FIG. 3 shows the variation of heterodyne signals H over time, in, forexample, three measuring channels assigned to three different measuringdimensions M1, M2, M3 in a time-multiplex heterodyne method. Heterodynesignals H appear in separate time windows and can be unambiguouslyassigned to measurement directions M1, M2, M3 during evaluation due tothese separate time windows. For example, the durations of the measuringtime windows can be on the order of μs. FIG. 4 shows, for example,modulated injection current I_(i) for the second measurement dimensionM2. Heterodyne signal H for second measurement dimension or secondmeasurement direction M2 is therefore formed during the time of therectangular pulse. The rectangular shape of injection current I_(i) isobtained according to the first operating mode shown in FIG. 1, with theminimum injection current I₀ being less than threshold current I_(th) inorder to prevent radiation for this measurement direction M2 beingemitted during the pauses, so that the signal can be unambiguouslyassigned to measurement direction M2 in a simple manner.

In a measurement system for the time multiplex heterodyne methodaccording to FIGS. 3 and 4, separate laser diodes are assigned to eachmeasurement direction M1, M2, M3 or each measuring channel; these diodesare controlled with the respective injection currents I_(i) usingtime-shifted pulses according to the time multiplex mode. Only onephotoelectric transducer, for example, a photodiode, must be providedfor the measurement receiver; the output signal of the photoelectrictransducer is assigned to the different measurement directions M1, M2,M3 according to the time multiplex operating mode.

Such a time multiplex heterodyne method can be advantageously used for aplurality of measurement directions in a multi-axis vibrometer with thedimensions of the device being kept extremely small.

Simultaneous measurement of a plurality of physical quantities can alsobe performed using the time multiplex heterodyne method in otherapplications, for example, in a 2-lambda interferometer for shapemeasurement. Only the appropriate number of measurement channels must beprovided.

What is claimed is:
 1. A method for interferometrically measuring aposition of a part, a position change of the part and a physicalquantity of the part, the physical quantity being derived from theposition and the position change, the part being tested using aheterodyne interferometry procedure, the method comprising the steps of:with a time-variable pulsating injection current, modulating a laser tochange a frequency of a radiation which is emitted by the part forgenerating a heterodyne frequency; routing a first portion of theemitted radiation via an optical bypass to the part; routing a secondportion of the emitted radiation without the optical bypass to the part;routing the second portion from the part to a measuring receiver,wherein a shape of a signal of the injection current has a rising edge,the edge being steeper as compared to a pulse length and a subsequentplateau of the signal; converting the signal in a photoelectrictransducer of the measuring receiver; and evaluating the convertedsignal only after a predetermined time period following an occurrence ofthe edge of the injection current during the subsequent plateau of thesignal.
 2. The method according to claim 1, wherein the signal has oneof a trapezoidal shape, a sinusoidal shape at edges, and a substantiallyrectangular shape.
 3. The method according to claim 1, wherein theinjection current pulsates between a minimum value and a maximum value,the minimum value being less than a threshold current of the laser, themaximum value being greater than the threshold current.
 4. The methodaccording to claim 1, wherein the injection current pulsates between aminimum value and a maximum value, the minimum value being greater thana threshold value of the laser, the maximum value being greater than theminimum value.
 5. The method according claim 1, further comprising thesteps of: correlating a plurality of signals in a time multiplex mode toform a plurality of measurement channels; and generating heterodynesignals of the measurement channels in non-overlapping time windows. 6.The method according to claim 5, further comprising the steps of:generating laser control signals for the measurement channels;delivering the laser control signals using a common control circuit;assigning laser diodes to each of the measurement channels; providingthe photoelectric transducer which corresponds to the measurementchannels; tapping off the signals from the photoelectric transducer inaccordance with the time multiplex mode; and separately evaluating thesignals for each of the measurement channels in a downstream evaluationcircuit.
 7. The method according to claim 5, further comprising the stepof: assigning the measurement channels to different dimensions.
 8. Avibrometer for interferometrically measuring a position of a part, aposition change of the part and a physical quantity of the part, thephysical quantity being derived from the position and the positionchange, the part being tested using a heterodyne interferometryprocedure, the vibrometer comprising: an optical bypass; laser diodes; acommon control circuit; a downstream evaluation circuit; a measuringreceiver including a photoelectric transducer; and an arrangement, usinga time-variable pulsating injection current, modulating a laser tochange a frequency of a radiation which is emitted by the part forgenerating a heterodyne frequency, the arrangement routing a firstportion of the emitted radiation via the optical bypass to the part, thearrangement routing a second portion of the emitted radiation withoutthe optical bypass to the part, the arrangement routing the secondportion from the part to the measuring receiver, wherein a shape of asignal of the injection current has a rising edge, the edge beingsteeper as compared to a pulse length and a subsequent plateau of thesignal, wherein the photoelectric transducer converts the signal, thearrangement evaluating the converted signal only after a predeterminedtime period following an occurrence of the edge of the injection currentduring the subsequent plateau of the signal, wherein a plurality ofsignals are correlated in a time multiplex mode to form a plurality ofmeasurement channels, heterodyne signals of the measurement channelsbeing generated in non-overlapping time windows, wherein the arrangementgenerates laser control signals for the measurement channels anddelivers the laser control signals using the common control circuit, thelaser diodes being assigned to each of the measurement channels, thephotoelectric transducer which corresponds to the measurement channelsbeing provided, wherein the signals are tapped off from thephotoelectric transducer in accordance with the time multiplex mode, thesignals being separately evaluated for each of the measurement channelsin the downstream evaluation circuit.